The protobuf code creates lots of spurious warnings
throughout our compile which we can do nothing about.
This patch changes the way we integrate the generated
code in our build to work around this.
We now run protoc and generated code compilation as a seperate
mumble_proto target producing a static lib. This lib is then
linked by mumble and murmur.
The folder of this module is added to the include paths so
Mumble.pb.h is available. On Unix platforms a -isystem entry
is added for the directory.
The advantages of this approach is that we can disable warnings
for the compilation of this target like we do for 3rd party modules.
Additionally we no longer generate and compile the protobuf files
twice.
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